1. Field of the Invention
The present invention relates to an optical transmitting apparatus and an optical receiving apparatus.
2. Background Art
Among optical communication techniques in recent years, wavelength division multiplex (WDM) is becoming widespread in which optical signals of different wavelengths are carried on one optical fiber so that optical signals of a plurality of channels are transmitted using one optical fiber. For example, as disclosed in Japanese Patent Laid-Open No. 2010-267734, there are known an optical transmitting apparatus and an optical receiving apparatus which multiplex or demultiplex laser beams of a plurality of wavelengths using a wavelength filter.
The wavelength filter is generally a prism provided with a wavelength filter film (dichroic coat in the above technique). According to the technique described in the above publication, the wavelength filter is mainly configured using not a parallel flat prism but a triangular prism. FIG. 3 in this document also discloses an optical transmitting apparatus using a parallel flat prism.
Although not related to optical communication, Japanese Patent Laid-Open No. 2010-267734 discloses an optical pickup for an optical disk drive. According to this publication, an optical path synthesizing prism is formed by applying a dichroic coat to a prism having an inclined surface.
Other prior art includes Japanese Laid-Open Patent Publication No. 2000-251312, Japanese Laid-Open Patent Publication No. H2-54207, Japanese Laid-Open Patent Publication No. S61-205906, Japanese Laid-Open Patent Publication No. 2004-128058, and Japanese Laid-Open Patent Publication No. 2002-185075.
Requirements for a wavelength filter for realizing optical communication include a requirement for increasing an optical path changing angle and a requirement for reducing an angle of incidence of a filter.
More specifically, first, the requirement for increasing an optical path changing angle is as follows. The “optical path changing angle” defined here for convenience is an angle related to positional relationships when an optical element, wavelength filter and optical fiber or the like are arranged. The optical element is a semiconductor laser for an optical transmitting apparatus or a photodiode for an optical receiving apparatus. The optical path changing angle is, more specifically in the case of a transmitting apparatus, an angle formed by an optical axis of a laser beam incident on a wavelength filter for multiplexing and an optical axis of a multiplexing laser beam emitted from the wavelength filter. In the case of an optical receiving apparatus, the optical path changing angle is an angle formed by an optical axis of a laser beam after wavelength demultiplexing to be received by a photodiode and an optical axis of a laser beam incident on the wavelength filter before wavelength demultiplexing.
When this optical path changing angle is too small, the optical axis of the laser beam incident on the wavelength filter is too close to the optical axis of the laser beam emitted. In this way, the optical fiber or the like and the optical element are aligned on the same line. An apparatus for performing wavelength division multiplexed communication is made up of a plurality of optical elements, a wavelength filter and an optical fiber aligned side by side. Decreasing the optical path changing angle involves a problem that it is difficult to accommodate the respective components in a compact space while avoiding mutual interference.
On the other hand, the requirement for reducing the angle of incidence of the filter is as follows. The angle of incidence of the filter is an angle of incidence of a laser beam with respect to the wavelength filter. Transmission characteristics of the wavelength filter are greatly dependent on the angle of incidence of the filter. To be more specific, regarding transmission characteristics of the wavelength filter, the smaller the angle of incidence of the filter, the more advantageous the angle tolerance and wavelength tolerance become. Especially in LAN-WDM or DWDM compared to CWDM, since the wavelength differences between a plurality of neighboring laser beams become smaller, more stringent specifications are required for the transmission characteristics of the wavelength filter. Therefore, the angle of incidence of the filter is required to be minimized.
The conventional optical communication apparatus disclosed in Japanese Patent Laid-Open No. 2010-267734 uses a triangular prism or parallel flat prism, but it has been unavoidable to increase the angle of incidence of the filter.